Underpotential depositon of metal monolayers from ionic liquids

ABSTRACT

A metal article comprises an alloy substrate having a surface and a non-diffused metal monolayer disposed thereon. The surface has a first surface work function value Φ s . The non-diffused monolayer deposited on the surface has a second surface work function value Φ s  that is less negative than the first surface work function value. A method for depositing the monolayer via underpotential deposition (UPD) is also disclosed.

BACKGROUND

The application relates generally to coating of metallic substrates, andmore specifically to applying a nondiffused metal coating to an alloysubstrate.

Bulk metal coatings have previously been applied directly ontosubstrates by electrically plating the substrate using an aqueouselectrolyte solution, often referred to as electrolytic deposition,electrodeposition, or electroplating. Coatings of more active metalsprovide protection to the base materials by sacrificial corroding of thecoatings, thus have been used extensively. Specifically, an aluminumcoating has been shown to be a drop-in replacement of cadmium to protecthigh strength steel, which is susceptible to hydrogen embrittlement whenit is protected by zinc or zinc alloys electroplated in an aqueousplating bath. The only commercial Al electroplating technology in theU.S. is Alumiplate™, which employs a bath that is pyrophoric(triethlyaluminium in toluene) and operates above room temperature (at100° C.). Such Al electroplating can be difficult and dangerous toimplement due in some part to the pyrophoric nature of the platingchemistry and use of organic solvents such as toluene. Toluene iscurrently listed by the U.S. Environmental Protection Agency (EPA) as ahazardous air pollutant (HAP). The process is also complicated orrendered incompatible for many combinations of coatings and substrates.

Other more advanced coating processes have been developed but each haveshortcomings. Thin film chemical vapor deposition (CVD) and physicalvapor deposition (PVD) can often be used for more precise control, butcannot be readily scaled up to larger industrial processes, nor is it apractical substitute for bulk deposition of thicker coatings. Recentadvances in ionic liquids and related processes have shown promise fordepositing metallic coatings directly onto a substrate. Many suchcoatings are otherwise difficult, dangerous, or impossible to depositusing aqueous electrolytes, e.g. Al, Ti and W coatings. These metals areabundant and excellent for corrosion resistance. However, problems havearisen with ensuring consistent chemical and structural compatibilityand adhesion between substrates and the bulk metal layer.

SUMMARY

A method for coating a metal article comprises providing an alloysubstrate, engineering at least one surface to be coated on the alloysubstrate, each of the at least one surfaces having a surface workfunction value Φ_(s). An ionic liquid deposition solution is formedcontaining a precursor of a depositing species having a work functionvalue Φ_(d) that is less negative than the surface work function Φ_(s).A monolayer is deposited via underpotential deposition (UPD) bycathodically reducing the precursor of the depositing species from theionic liquid deposition solution onto the surface of the alloysubstrate, the deposited monolayer having a depositing work functionvalue Φ_(d) that is less negative than the surface work function valueΦ_(s).

Optionally, the method further comprises depositing a bulk layer ontothe monolayer after the UPD depositing step.

When the method further comprises depositing a bulk layer onto themonolayer after the UPD depositing step, the monolayer and the bulklayer are optionally both substantially pure aluminum.

When the method further comprises depositing a bulk layer onto themonolayer after the UPD depositing step, and when the monolayer and thebulk layer are both substantially pure aluminum, the method optionallyfurther comprises anodizing at least a portion of an outer surface ofthe bulk aluminum layer after the bulk depositing step.

The engineering step of the method optionally includes increasing firstsurface work function value Φ_(s) by selectively configuring the alloysubstrate such that the surface to be coated has a surface work functionvalue Φ_(s) greater than work function values of at least one surface ofthe alloy substrate that is not to be coated by the monolayer.

When the engineering step of the method includes increasing firstsurface work function value Φ_(s) by selectively configuring the alloysubstrate such that the surface to be coated has a surface work functionvalue Φ_(s) greater than work function values of at least one surface ofthe alloy substrate that is not to be coated by the monolayer, the alloysubstrate optionally has a directionally solidified face centered cubic(FCC) microstructure.

When the engineering step of the method includes increasing firstsurface work function value Φ_(s) by selectively configuring the alloysubstrate such that the surface to be coated has a surface work functionvalue Φ_(s) greater than work function values of at least one surface ofthe alloy substrate that is not to be coated by the monolayer, and whenthe alloy substrate has a directionally solidified face centered cubic(FCC) microstructure, the surface to be coated optionally runssubstantially along the (100) plane of the FCC microstructure.

When the engineering step of the method includes increasing firstsurface work function value Φ_(s) by selectively configuring the alloysubstrate such that the surface to be coated has a surface work functionvalue Φ_(s) greater than work function values of at least one surface ofthe alloy substrate that is not to be coated by the monolayer, when thealloy substrate has a directionally solidified face centered cubic (FCC)microstructure, and when the surface to be coated runs substantiallyalong the (100) plane of the FCC microstructure, the alloy substrateoptionally includes a majority by weight of aluminum.

The engineering step optionally includes cleaning the surface to becoated in a substantially oxygen-free atmosphere and maintaining the atleast one surface to be coated in a substantially oxygen-free atmospherethrough the depositing step.

The UPD depositing step optionally includes increasing the voltage in astepwise fashion to form a plurality of monolayers.

The optional stepwise voltage increase to form a subsequent monolayeroptionally occurs after substantially complete deposition of a precedingmonolayer.

Optionally, the method further comprises depositing an interlayer ontothe surface to be coated prior to the monolayer depositing step.

When the method further comprises depositing an interlayer onto thesurface to be coated prior to the monolayer depositing step, theinterlayer optionally has a work function value intermediate between awork function value of the surface to be coated and a work functionvalue of the monolayer.

A metal article comprises an alloy substrate including a surface to becoated and a non-diffused metal monolayer disposed thereon. The surfacehas a first surface work function value Φ_(s). The non-diffusedmonolayer deposited on the surface has a second surface work functionvalue Φ_(s) that is less negative than the first surface work functionvalue.

Optionally, the article further comprises a bulk layer deposited atopthe monolayer.

The surface of the alloy substrate is optionally substantially free ofsurface adsorbed impurities between the substrate and the monolayer.

The surface of the alloy substrate optionally has a selectivelyconfigured lattice orientation for increasing a first surface workfunction value Φ_(s) as compared to non-coated surfaces of the alloysubstrate.

When the surface of the alloy substrate has a selectively configuredlattice orientation for increasing a first surface work function valueΦ_(s) as compared to non-coated surfaces of the alloy substrate, thealloy substrate optionally has a directionally solidified face centeredcubic (FCC) microstructure.

When the alloy substrate has a directionally solidified face centeredcubic (FCC) microstructure, the alloy substrate optionally includes amajority by weight of aluminum.

When the alloy substrate has a directionally solidified face centeredcubic (FCC) microstructure and the alloy substrate includes a majorityby weight of aluminum, the at least one surface to be coated isoptionally configured to be primarily along (100) lattice planes of thealuminum alloy substrate.

The monolayer optionally consists of substantially pure aluminum.

-   -   Optionally, the article further comprises an interlayer disposed        between the surface of the alloy substrate and the monolayer,        the interlayer being a different metal than the monolayer and a        base element of the alloy substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a cross-section of a coated metal article having anengineered substrate surface, a monolayer and a bulk deposition layer.

FIG. 2A depicts a first microstructure of an engineered substratesurface prior to deposition of the monolayer.

FIG. 2B depicts a second microstructure of an engineered substratesurface prior to deposition of the monolayer.

FIG. 2C depicts a third microstructure of an engineered substratesurface prior to deposition of the monolayer.

FIG. 3 shows an alternative coated metal article with an interlayerdisposed between the monolayer and the engineered substrate surface.

FIG. 4 is a flow chart depicting the steps of the coating process.

DETAILED DESCRIPTION

FIG. 1 includes article 10, alloy substrate 12, engineered substratesurface 14, coating 16, monolayer 18, bulk deposition layer 20, andcoating outer surface 21. Coated article 10 can be made for any numberof applications requiring a coating a surface of an alloy substrate, andthus the choice of material for substrate 12 and coating 16 will besuitable for the chosen application. In the illustrative example,substrate 12 is an aluminum alloy and coating 16 is substantially purealuminum. In certain aluminum based embodiments, article 10 can be usedin various structural aircraft components as well as for cold sectionuse in gas turbine engines.

Alloy substrate 12 includes engineered substrate surface 14 with coating16. Coating 16 includes monolayer 18 and bulk deposition layer 20.Monolayer 18 is a very thin layer substantially contiguous with thelattice of the alloy substrate, and can be as thin as a single atomicunit cell. Monolayer 18 is applied so as to act as a non-diffused stable“seed” or “bonding” surface for bulk deposition layer 20. Monolayer 18thus can strengthen the bond between substrate 12 and bulk depositionlayer 20 without diffusing the coating into the substrate, which alsosimplifies repair and refurbishment of article 10. As will be explained,in certain embodiments both monolayer 18 and bulk deposition layer 20can be the same metal, such as substantially pure aluminum. In the caseof aluminum coating 16, an outer surface 21 of bulk layer 20 mayoptionally be anodized or otherwise protected according to traditionalmethods.

In this example, non-diffused monolayer 18 is deposited first ontoengineered substrate surface 14 via underpotential deposition (UPD) froman ionic liquid deposition solution. UPD refers to depositing at least athin layer on the surface of a substrate at a potential less negativethan the Nernst (equilibrium) potential required for bulk deposition. Aswill be seen, this provides a stable non-diffused base for the bulkcoating. Bulk coatings have been traditionally applied directly to anarticle via aqueous electrodeposition. Some plating materials likealuminum and titanium cannot be electrodeposited in bulk from aqueoussolutions. Water from the aqueous solution can dissociate into hydrogenand oxygen ions at a voltage lower than what is necessary for the metalcation to reduce out of solution to its metallic state. In other cases,substrates like high strength steel are prone to hydrogen embrittlement.Al plating from ionic liquids offers a promising solution to theseissues, but the challenge is the Al film coating adhesion to thesubstrate. Even when materials and conditions are suitable for bulkdepositing a coating, coating bond strength still depends on substratestructure and cleanliness, agitation, temperature, aqueous electrolytechemistry, and modulation of the electric field during plating.

Film deposition techniques have been developed to more finely controlplacement of very thin coatings. However, existing thin film depositiontechniques like chemical vapor deposition (CVD), physical vapordeposition (PVD), flame spray, laser deposition, etc. are limited toline of sight applications, and regardless of this, the processes cannotbe economically scaled up to form useful bulk coatings. In addition,conventional thin film deposition still presents some difficulty andinconsistency in forming a thin film layer onto the substrate,particularly thin films that are substantially coherent with thesubstrate lattice. While more precise than direct bulk deposition, mostfilm deposition processes rely almost exclusively on controlling theenergy and throwing power of the deposition solution, not the substrateor deposition surface thereon. Thus due to surface effects and differentalloy phases on the substrate surface under more conventional thin filmdeposition, the layer often ends up being discontinuously depositedrelative to the substrate lattice, undermining the bond between thesubstrate and the bulk deposition layer. For these and other reasons, toimprove coating adherence, incompatible surface energies thereforerequire that the thin film and/or the bulk layer be at least partiallydiffused into one another so as to commingle and transition the latticeparameters of the substrate into the coating. However, diffusing thecoating into the substrate adds process steps and also complicates laterremoval and repair of the coating and substrate. Other considerationsmay indicate diffusion of all or part of coating 16 into the substrateafter deposition of coating 16, but such considerations are outside thescope of this disclosure.

In contrast, coating 16 can be effectively bonded to substrate 12 usingmonolayer 18. UPD application of monolayer 18 facilitated by engineeringsurface 14 to increase its work function values Φ_(s), improvesadherence and repair of coating 16. The theoretical work function valueΦ₀ of a metal surface is defined as the energy required to remove anelectron to a location immediately away from the metal surface. Leavingout complicating surface effects, theoretical work function value Φ₀ isroughly equivalent to the negative of the Fermi energy plus theeffective surface dipole. An electron's Fermi energy is a measure of howstrongly the electrons in outer valence bands of the metal are retainedto the substrate. (i.e., minimum energy of a bound electron at thehighest occupied quantum state between valence and conduction bands).The surface dipole represents energy possessed by surface electrons dueto lattice discontinuities.

It should be noted that the theoretical work function value Φ₀ is basedon assumptions of an infinite surface in a vacuum at absolute zerotemperature, and thus a real life metal substrate will have differentwork function values Φ_(s) for different surfaces and even differentregions on each surface. A number of factors greatly influence empiricalwork function values Φ_(s). By way of non-limiting examples, any changein adsorption, surface morphology, composition, etc. all can change thevalue of the work function. UPD works with engineered surface 14 becausehigher work function values Φ_(s) represent increased affinity forelectrons toward surface 14. This makes it possible to lower theelectrical potential (as compared to the that must be applied to theionic liquid deposition solution in order to electrochemically reducecations (precursors) of the depositing species out of the ionic liquiddeposition solution onto surface 14. Engineering of surface 14 toincrease its work function value Φ_(s) for facilitating UPD is discussedbelow.

FIG. 2A shows surface 14A with substrate atoms 22, voids 24, anddepositing adatom 30. FIG. 2B has surface 14B with substrate atoms 22,voids 26, and depositing adatom 30. FIG. 2C shows surface 14C withsubstrate atoms 22, voids 28, and depositing adatom 30. FIG. 2A is aview of face centered cubic (FCC) lattice taken across the (100)crystallographic plane. FIG. 2B shows the FCC lattice taken across the(110) crystallographic plane. FIG. 2C is the FCC lattice taken acrossthe (111) crystallographic plane.

TABLE 1 Metal Crystal structure Φ_(s) (111) Φ_(s) (100) Φ_(s) (110) AlFCC 4.24 4.42 4.28 Ag FCC 4.56 4.42 4.35 Au FCC 5.26 5.22 5.20 Cu FCC4.94 4.59 4.48 Ir FCC 5.76 5.67 5.42 Cr FCC ~4.50 ~4.50 ~4.50 Ni FCC5.25 4.95 4.55 W BCC 4.47 4.63 5.25

Microstructures, lattice parameters, and Miller indices are known in theart and their details will not be repeated here. In one example ofengineering surface 14 to provide a suitable work function Φ_(s),article 10 (shown in FIGS. 1A and 1B) can be configured such thatsurface 14 substantially coincides with one or more of surfaces 14A,14B, 14C having the highest work function value Φ_(s). Table 1 abovelists work function values Φ_(s) for surfaces having differentcrystallographic orientations. As is already known, the (111)crystallographic plane has the closest packing of atoms in an FCClattice thus leaving the smallest voids 28 between atoms 22. Thisarrangement is also referred to in the art as a cubic close packed (ccp)structure. Open surfaces such as (100) and (110) surfaces respectivelyhave larger voids 24, 26 as seen in FIGS. 2A and 2B. As seen in Table 1,work function values Φ_(s) for many FCC metals are higher on theclose-packed (111) surface as compared to Φ_(s) seen on the more open(100) and (110) surfaces. This is because for many FCC metal substratesexcept for aluminum (such as Ag, Au, Cu, Ir, Ni, and their alloys), opensurfaces usually exhibit the “Smoluchowski” effect of charge smoothing,where a dipole moment develops around atoms 22. This charge smoothingdipole opposes the dipole created by flow-out of electrons, resulting intransfer of surface charge into voids or interstices between atoms (e,g,voids 24 and/or 26). This effect lowers the work function value on manyopen FCC surfaces, making UPD more difficult as compared to the closepacked surfaces of the same alloys.

In contrast to these other FCC alloys, open surfaces of aluminum andmost of its common alloys do not strongly exhibit the Smoluchowskieffect. As seen in Table 1, the close packed (111) surfaces of aluminumand its alloys have the lowest work function value Φ_(s) as compared totheir more open (100) and (110) surfaces. So in this example, substrate12 can be prepared with engineered surface 14 that is co-located withthe (100) and/or (110) surface, where the crystallographic orientationis more favorable to UPD with a higher work function value Φ_(s).

Just as importantly, it should be noted that aluminum has overall lowerwork function values than the other example materials listed in thetable. While this complicates adhesion of a pure aluminum coating toother substrates using conventional bulk and thin film techniques, italso makes aluminum an excellent candidate for UPD of aluminummonolayers onto both aluminum and non-aluminum based substrates withsuitably engineered work function values. The UPD shift (ΔUPD) away fromthe Nernst bulk deposition potential can be theoretically estimatedaccording to Equation 1:

ΔUPD˜0.5*(Φ_(s)−Φ_(d))  [1]

In other words, the UPD shift is approximately half of the differencebetween the first surface work function value Φ_(s) and the workfunction value Φ_(d) of the monolayer formed by the depositing species(adatoms 30). When the surface work function value Φ_(s) is higher (morenegative) than the depositing species work function value Φ_(d), theadatom has a higher affinity for the substrate than for other atoms inthe solution, and thus a limited number of these precursors can becathodically reduced out of solution and onto engineered surface 14 toform monolayer 18. Once deposited, monolayer 18 will then have a lessnegative second outer surface work function value Φ. As can be seen inTable 1, substantially pure aluminum works well for monolayer 18 on bothaluminum alloys as well as other alloys because of relatively largedifferences in work function values Φ_(s) compared to other candidates.

Regardless of the effect on aluminum alloy substrates, it will beappreciated that in the cases of other non-aluminum alloy substrates,substrate 12 can be engineered such that surface 30 to be coated is thesurface with a higher work function that is favorable to UPD. Forexample, in the case of a BCC alloy like tungsten, a higher workfunction value Φ_(s) is available on the (110) surface as compared toother surfaces. It will be recognized that substrate 12 can beengineered to have the selected crystallographic plane by formingsubstrate 12 either as a directionally solidified or a single crystalalloy casting. Surface 14 can then be a surface of the casting or can befurther processed (e.g. machined) into its shape prior to applyingcoating 16.

In another example of engineering surface 14 to increase work functionvalue Φ_(s), surface 14 can be subjected to specialized treatments. Onetreatment prevents adsorption of oxygen onto the surface. Due to theirhigh electron affinity and larger size voids 24, 26 on respectivesurfaces 14A, 14B are particularly reactive to oxygen. Additionally,other reactive atmospheric contaminants can also bond to surface 14,which can cause voids 22, 24, and/or 26 to become saturated fromreactively adsorbing these gases. If not mitigated, this lowers workfunction value Φ_(s), reducing effectiveness of UPD on surface 14.

In the example of aluminum and aluminum alloys, this issue can bemitigated by surface treatment using a deaerated cleaning solution. Inone example, the cleaning solution can be deaerated by bubbling agaseous mixture through the solution which consists almost exclusivelyof nitrogen and argon. The argon substantially replaces atmosphericoxygen. In one of these examples, argon comprises about 15 and about 22mol % of the mixture, with nitrogen substantially comprising theremainder, apart from incidental elements and contaminants. Under asubstantially oxygen-free atmosphere, the surface can be cleaned using adeaerated 0.5M to 1M aqueous hydrochloric acid (HCl) or nitric acid(HNO₃) solution. The substrate should be maintained in an oxygen freeenvironment, such as using the example atmosphere just described, Otherexample processes for cleaning and activating an aluminum alloy surfaceare described in a commonly assigned United States patent applicationentitled “Method for Surface Cleaning and Activation”, filed on an evendate herewith, the entirety of which is herein incorporated byreference.

Chemistry of the ionic liquid deposition solution can also be controlledto decrease Φ_(d) as well as prevent surface contamination orpassivation. In the example of aluminum monolayer deposition, a simplealuminum salt such as aluminum chloride (AlCl₃) is mixed with an ionicliquid solvent. In certain embodiments, the ionic liquid solvent is aform of methylimidazolium chloride. In certain of those embodiments, theionic liquid comprises at least one of: 1-ethyl-3-methylimidazoliumchloride, 1-butyl-3-methylimidazolium chloride,1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide,1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)amide,trihexyl-tetraadecyl phosphonium bis(trifluoromethylsulfonyl)amide, andmixtures thereof. To further facilitate UPD, the work function ofaluminum cations in the ionic liquid deposition solution can beincreased. In this example, a second solvent or a surfactant can beadded to the mix in order to further dissociate or “wet” the aluminumcations, lowering their work function Φ_(d) and making the aluminumcations more susceptible to deposition upon applying the UPD voltage.Suitable compounds for improving dissociation of aluminum cations caninclude, but are not limited to sodium dodecyl sulfate (SDS) andtrimonium chloride. Such surfactant compounds can be added to thesolution at or above their critical micelle concentration (CMC) at whichpoint the cations and anions are most easily separated. The surfactantalso attracts and captures many potential surface contaminantspreventing a decrease in the surface work function Φ_(s) during UPD.Exact surfactant concentrations and the actual CMC threshold will dependon factors like the concentration and choice of the simple metal saltrelative to the ionic liquid solvent.

In the presence of the chosen ionic liquid solvent, aluminum cations(Al³⁺) readily dissociate from the halide anions, here chloride (Cl⁻).Appropriate voltage is then applied through the solution between ananode (not shown) and a cathode (surface 14). This voltage generally isjust above the equilibration potential between the depositing cationsand the reduced metal. In one illustrative example utilizing Al³⁺cations reduced to an Al coating, this figure can be on the order ofabout 50 mV to about 100 mV above the equilibration potential. Thisfigure will depend on the particular choice and concentration ofdepositing cations, solution temperature, and actual cathode surfacework function values Φ_(s). To deposit a plurality of monolayers, thevoltage can be slowly increased in a stepwise fashion after eachindividual monolayer is completely formed. The first step application ofvoltage will result in a single monolayer. The stepwise voltage increasefor each subsequent monolayer will initially be less than the initialincrease to form the first monolayer, but each step will beprogressively larger. Once monolayer 18 has reached an appropriatethickness, voltage is stopped and substrate 12 is removed from thesolution and prepared for bulk deposition.

Upon formation of the one or more monolayers 18, bulk layer 20 is thendeposited. This can be done from an ionic liquid, or by traditional bulkdeposition techniques. It will of course be recognized that if doneusing a separate tank or other vessel, aluminum monolayer 18 will haveto be protected or cleaned during transfer to the bulk deposition vesselso as to prevent or remove oxide buildup. This can be done for exampleby maintaining plating vessels and the path between them in an oxygenreduced or oxygen free atmosphere similar or identical to one discussedabove, which will reduce or prevent oxygen contact with the high workfunction surface. It will also be recognized that after bulk layerdeposition, coating 16 (shown in FIGS. 1A and 1B) can be furtherprocessed for protection such as by surface anodization.

Prior to bulk layer 20 deposition, the one or more monolayers 18 will beapplied to be thick enough so as to provide an adequate bonding surfacefor bulk layer 20. This will depend on the lattice compatibility ofalloy substrate 12 and bulk coating 20, where more similar latticeparameters generally reflecting the use of fewer monolayer 18. The oneor more applied monolayers 18 can be as thin as only a few atomiclayers, and will depend in part on the size and packing density of atomsbeing deposited. In certain embodiments, the thickness of the one ormore monolayers 18 is on the order of between about 0.1 nm and about 2.0nm. In certain of those embodiments, monolayer 18 can be as thick asabout 1.0 nm prior to deposition of bulk coating layer 20. Once bonded,monolayers 18 have a high stripping potential as compared to a bulklayer alone, improving adhesion and integrity of coating 16. It alsoeliminates the need to diffuse coating 16 into substrate 12 whichsimplifies repair of article 10 by minimizing the quantity and depthmaterial that must be removed and replaced.

FIG. 3 shows an alternative embodiment with interlayer 118 disposedbetween substrate surface 14 and monolayer 18. In the earlier describedexample embodiments, monolayer 18 is the same material as bulk layer 20,as well as being the same as the base element in the alloy definingsubstrate 12 (e.g., monolayer 18 and bulk layer 20 are aluminum whensubstrate 12 is an aluminum alloy. However, in some instances such aswhen there is a different base element for substrate 12, or when thealloy has a sufficiently different lattice parameter as compared to thatof monolayer 18, interlayer 118 can be deposited between the monolayerand the substrate to provide any required work function transitiontherebetween. In one such example, substrate 12 is a nickel-based alloy,monolayer 18 and bulk layer 20 are aluminum, while interlayer 118 ischromium or a chromium-based alloy. This can be seen from Table 1 wherechromium (Cr) has an intermediate work function between that of nickel(Ni) and aluminum (Al). Surface 14 can be engineered to increase oroptimize its work function value as shown above. Here, interlayer 118 isdeposited using a traditional thin film process, such as CVD, PVD, laserdeposition or flame spray, while monolayer 18 is deposited ontointerlayer 18 from an ionic liquid solution as described above.Interlayer 118 can also be deposited via UPD.

FIG. 4 is a flow chart of the process 100. At step 102, an alloysubstrate having at least one surface to be coated is provided. Step 104includes engineering at least one of the surfaces to be coated so as tohave its work function value sufficiently negative to have the at leastone surface operate as a cathode for underpotential deposition (UPD).This can be done according to one of the methods described above withrespect to FIGS. 1 and 2, including but not limited to arranging thecrystallographic orientation of the substrate such that the workfunction value Φ_(s) is higher than the other orientations, or isotherwise suitable for UPD. This portion of step 104 can be donesimultaneous to step 102, such as during casting. Alternatively apre-formed single crystal or directionally solidified body of materialcan be processed so as to form surface(s) 14 on the correct plane(s). Asnoted above, the work function value Φ_(s) of the surface(s) canadditionally or alternatively be engineered by cleaning and processingthe surface(s) to be coated so as to remove and/or avoid atmosphericadsorption into voids between atoms at or near the surface.

Next, the ionic liquid deposition solution is formed at step 106. Atthis step, the solution includes the species to be deposited as themonolayer (such as aluminum). As noted above, the solution can be astandard ionic liquid deposition solution when surface 14 has beenengineered with a sufficient surface work function value Φ_(s). However,the work function value of the depositing species can be also optimizedto further facilitate UPD by adding a second solvent or a surfactantsuch as SDS or trimonium chloride. This can be done in amounts such thatthe CMC is reached or exceeded which improves dissociation between metalcations and halide anions as noted above. Example embodiments of thiscleaning process are detailed in the application incorporated byreference.

After step 106, optional step 108 includes depositing an interlayerprior to depositing the monolayer. The interlayer will be a differentmetal than either the base element of the alloy substrate and thespecies being deposited as the monolayer. Depending on the choice of theinterlayer, it can be deposited via UPD in a manner similar to applyingthe monolayer, or according to conventional film deposition techniques.

After step 106 and optional step 108, a monolayer is formed viaunderpotential deposition at step 110 by passing an electrical chargethrough the ionic liquid deposition solution between an anode and the atleast one engineered surface (acting as the cathode). As the monolayeris formed, the applied voltage (potential) of the substrate (cathode)vs. anode is less than would be required to otherwise reduce thedepositing species cations out of the ionic liquid deposition solution(the equilibrium potential). As noted previously, engineering thesurface to be coated decreases the required potential for reducing thecations from the solution, and is thus less negative than theequilibrium potential that would otherwise be required to reduce themetal cations out of the plating solution. As discussed above, duringthe UPD step, the voltage can be increased in a stepwise fashion todeposit a plurality of monolayers.

Finally step 112 is performed by depositing a bulk layer on themonolayer to complete the coating. Deposition of the bulk layer willdepend on the particular costs and benefits for each substrate andcoating, and can be by any suitable process. In the example of aluminum,bulk layer can be deposited either from an ionic liquid depositionsolution without a second solvent or surfactant, or can alternatively bedeposited by other conventional processes.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

1. A metal article comprising: an alloy substrate including a surface tobe coated having a first surface work function value Φ_(s); and at leastone non-diffused monolayer deposited on the surface of the alloysubstrate, the non-diffused monolayer having a second surface workfunction value Φ_(d) that is less negative than the first surface workfunction value.
 2. The article of claim 1, further comprising a bulklayer deposited atop the monolayer.
 3. The article of claim 1, whereinthe surface of the alloy substrate is substantially free of surfaceadsorbed impurities between the substrate and the monolayer.
 4. Thearticle of claim 1, wherein the surface of the alloy substrate has aselectively configured lattice orientation for increasing a firstsurface work function value Φ_(s) as compared to non-coated surfaces ofthe alloy substrate.
 5. The article of claim 4, wherein the alloysubstrate has a directionally solidified face centered cubic (FCC)microstructure.
 6. The article of claim 5, wherein the alloy substrateincludes a majority by weight of aluminum.
 7. The article of claim 6,wherein the at least one surface to be coated is configured to beprimarily along (100) lattice planes of the aluminum alloy substrate. 8.The article of claim 1, wherein the monolayer consists of substantiallypure aluminum.
 9. The article of claim 1, further comprising aninterlayer disposed between the surface of the alloy substrate and themonolayer, the interlayer being a different metal than the monolayer anda base element of the alloy substrate.
 10. A method for coating a metalarticle, the method comprising: providing an alloy substrate;engineering a surface of the alloy substrate to be coated to have asurface work function value Φ_(s); forming an ionic liquid depositionsolution containing a precursor of a depositing species; and depositinga monolayer via underpotential deposition (UPD) by cathodically reducingthe precursor of the depositing species from the ionic liquid depositionsolution onto the surface of the alloy substrate, the depositedmonolayer having a depositing work function value Φ_(d) that is lessnegative than the surface work function value Φ_(s);
 11. The method ofclaim 10, further comprising after the UPD depositing step: depositing abulk layer onto the monolayer.
 12. The method of claim 11, wherein themonolayer and the bulk layer are both substantially pure aluminum. 13.The method of claim 12, further comprising after the bulk depositingstep: anodizing at least a portion of an outer surface of the bulkaluminum layer.
 14. The method of claim 10, wherein the engineering stepincludes increasing first surface work function value Φ_(s) byselectively configuring the alloy substrate such that the surface to becoated has a surface work function value Φ_(s) greater than workfunction values of at least one surface of the alloy substrate that isnot to be coated by the monolayer.
 15. The method of claim 14, whereinthe alloy substrate has a directionally solidified face centered cubic(FCC) microstructure.
 16. The method of claim 15, wherein the surface tobe coated runs substantially along the (100) plane of the FCCmicrostructure.
 17. The method of claim 16, wherein the alloy substrateincludes a majority by weight of aluminum.
 18. The method of claim 10,wherein the engineering step includes cleaning the surface to be coatedin a substantially oxygen-free atmosphere and maintaining the at leastone surface to be coated in a substantially oxygen-free atmospherethrough the depositing step.
 19. The method of claim 10, wherein the UPDdepositing step includes increasing the voltage in a stepwise fashion toform a plurality of monolayers.
 20. The method of claim 19, wherein thestepwise voltage increase to form a subsequent monolayer occurs aftersubstantially complete deposition of a preceding monolayer.
 21. Themethod of claim 10, further comprising prior to the monolayer depositingstep: depositing an interlayer onto the surface to be coated.
 22. Themethod of claim 21, wherein the interlayer has a work function valueintermediate between a work function value of the surface to be coatedand a work function value of the monolayer.